Earlier quoted context omitted.
Wouldn't this then require us to "fart around" with the climate of the planets we colonize to make them more like Earth's?
The difference being that there is nobody living on a planet like say, Mars. Last I checked, there are a whole bunch of people with nowhere else to go on Earth as it stands.
Stop right there. You don't know that there isn't anyone living there. We've sent a couple rovers, dug down a few inches, and looked at it from telescopes. There are millions of dollars poured into the search for life on Mars every year, as there should be. But we don't know very much yet!
I am totally in favour of terraforming and colonizing Mars if we can (reasonably) prove that nothing is alive there. But we aren't anywhere near that point. The greatest crime the human race could ever commit would be the annihilation of the (perhaps only?) living ETs found, even if by "accident" as we moved in there.
Don't think that Mars is your dead playground for you to do as you wish. Treat it and its unknown past with deep respect. There's a small chance that something, somewhere, is living there, perhaps under the surface, perhaps in caves.
Don't assume we know everything about other planets just 'cause we can see them with our telescopes.